The hazards of airborne asbestos fibers have been well documented, and have necessitated their replacement by other types of fibers in friction elements. Such replacement has been difficult for asbestos fibers are unique minerals combining unusual physical and chemical properties which have made them useful in the manufacture of a wide variety of products. Being of mineral origin, asbestos does not burn or rot, and, depending on variety, possesses extremely high tensile strength as well as resistance to heat. Because of this community of advantageous properties, asbestos fibers have been used extensively by the automotive industry in the manufacture of friction members such as disc brake pads, brake linings and clutch facings.
Asbestos fibers also have an unique hollow tubular structure. In the manufacture of friction elements the reinforcing fibers, particulate friction modifiers and inert fillers are bound together in a heat-curable binder which generally comprises a thermosetting resin, such as a phenol formaldehyde resin, and a heat-curable elastomer, frequently a styrene-butadiene rubber (SBR). During the curing process in which the binder is converted under heat and pressure to the infusible insoluble thermoset state, gases are generated and are vented by periodically opening the molds. By reason of the hollow tubular structure of asbestos fibers, friction elements containing such fibers permitted escape of such gaseous materials and thus there was no problem of forming pockets or "blisters" in the molded elements by entrapped gases. However, the fibers used to replace asbestos are substantially non-porous, and even if somewhat porous, the pores are not interconnected. As a result, in manufacture of friction elements containing such substantially non-porous fibers, gases have been entrapped within the body of the friction elements causing formation of blisters thereby resulting in defective products which could fail in use. Accordingly, such defective products have been treated as rejects. Of course, it is vital to the economic viability of any manufacturing process that the number of rejects be kept to an absolute minimum.
Not only must friction elements be free of defects such as blisters, they must also meet certain industry accepted standards. Major manufacturers of automotive clutch facings require that clutch facings meet a particular performance standard in the hot burst strength test. In this test, the driven member of the clutch, with two facings attached, is heated to 500.degree. F. (260.degree. C.) for 15 minutes and then is subjected to spin testing as a prerequisite to product acceptance. This test is one of the most difficult tests to be passed by a clutch facing since at elevated temperatures fibrous material and the binder comprising resin and/or rubber of which the clutch facing is formed lose strength, resulting in bursts at relatively low rotational speeds. Conventional extruded clutch facings formed of asbestos fibers having an OD of 11 inches (27.94 cm) and ID of 61/2 inches (16.51 cm) are known to withstand spin speeds averaging about 7,000 r.p.m. in the hot burst strength test.
It is an object of this invention to provide a novel extrusible asbestos-free composition for forming a friction member, especially a clutch facing, having improved properties.
Another object of this invention is the provision of novel asbestos-free clutch facings which are substantially free of blisters.
Still another object of this invention is to provide a novel, simple and relatively inexpensive method for making asbestos-free clutch facings and other non-asbestos friction members which are substantially free of blisters.
These and other objects of this invention will become further apparent from the following detailed description of the invention. Although such description relates specifically to the fabrication of clutch facings, it will be recognized by those skilled in the art that the compositions and methods can be used to make other types of friction members.